1. Field of the Invention
This invention relates to heat treatment of a metal strip, and particularly to the accurate temperature control of a moving metal strip in a continuous heat treating operation. The invention is especially useful in the production of dual phase steel strip by accurately controlling at the cooling rate of the heated strip following heat treatment in a continuous annealing oven.
2. Description of the Prior Art
In the manufacture of elongated or continuous metal articles such as metal strip, pipe, bars, plate and the like, it is conventional practice to employ controlled cooling at various stages such as during or following annealing or on a run-out table following a hot rolling or other hot forming operation. Various means have been employed to control the cooling rate, including measuring the temperature of the product as it enters the cooling area, determine the total temperature reduction required which, combined with the measured rate of movement through the cooling area, is utilized to determine the total cooling required. Application of cooling fluid is then increased or decreased along the entire cooling area in such a way that the final temperature of the product has always reached the desired lowered temperature regardless of the speed of the product or the temperature entering the cooling zone. This process, however, results in a higher cooling rate with increased product speed and/or increased temperature entering the cooling zone.
It is also known to provide a cooling zone, for example, a run-out table, having a total length greater than normally required to cool a product moving thereon, and to provide a plurality of cooling means located at spaced intervals along the length of the cooling zone. A computer or microprocessor is then used to control the activation and/or deactivation of selected ones of the groups of cooling devices along the length of the cooling zone depending upon measured product speed and the temperature of the product entering the cooling zone. This process, utilizing only a part of the cooling sections, is intended to obtain an approximation of the desired cooling rate, i.e., an average cooling rate for the product during the time required to pass through the cooling zone.
Other known cooling rate control systems have included utilizing a rolling mill control computer, on a time sharing basis, to monitor one or more spray cooling zones and to control the cooling in such zones based upon operating signals from various information sources including measured strip speed, thickness, and initial temperature, and from programmed information relating to desired or fixed data. A variation of this concept is to employ a fixed length of a cooling zone and varying the rate of flow of the cooling fluid in the zone based upon the comparison of measured values such as strip velocity and/or strip discharge temperature with the desired values.
While the known cooling systems have been adequate for cooling hot metals such as low carbon steel, these systems have generally not been capable of controlling the cooling rate with the desired degree of accuracy throughout the cooling zone where accurate control of the crystalline structure of the product is required. For example, the control of the cooling rate following annealing is critical in the production of the high-strength steel strip having exceptional formability and a ferrite-martensite, dual-phase, microstructure described in pending U.S. patent application Ser. No. 931,684 filed Aug. 7, 1978, now U.S. Pat. No. 4,159,218, assigned to the Assignee of the present invention. Accurate control of the cooling rate within the temperature range between the A.sub.1 and A.sub.3 transformation points, i.e., within the intercritical temperature range of about 1337.degree. F.-1616.degree. F. (725.degree. C.-880.degree. C.) down to the martensite forming temperature, for example 850.degree. F..+-.100.degree. F. (454.degree. C..+-.56.degree. C.) is critical in the production of a high-quality dual-phase strip in accordance with the invention disclosed in that prior application. Further, for the production of such high-quality dual-phase steel strip, it is important that the cooling rate be accurately controlled throughout the critical temperature range within relatively close tolerances. It is, therefore, a primary object of the present invention to provide an improved method of and apparatus for controlling the temperature of a moving product in a continuous heat treatment operation.
Another object of the present invention is to provide an improved method of and apparatus for accurately controlling the cooling rate of a heated metal product moving through a cooling zone.
Another object of the invention is to provide an improved method of and apparatus for accurately controlling the temperature of a continuous length of heated metal moving through a cooling zone to provide a substantially uniform cooling rate throughout the length of the cooling zone.
Another object of the present invention is to provide an improved method of and apparatus for producing a high-strength dual-phase ferrite-martensite steel strip of uniform high quality.